1. Field of the Invention
The present invention relates to an optotype presenting apparatus which presents an optotype and/or a test chart for testing visual acuity and binocular visual performance of eyes of an examinee.
2. Description of Related Art
Conventionally, there are known an optotype presenting apparatus such that an optotype and/or a test chart drawn on a disk is illuminated from behind with a light source such as a halogen lamp and the optotype and/or the test chart is projected onto a screen at a far distance for a test of such as 5 meters, and an optotype presenting apparatus having a display such as a color liquid crystal display (see Japanese Patent Application Unexamined Publication No. Hei 05-130975 corresponding to U.S. Pat. No. 5,638,082, and Japanese Patent Application Unexamined Publication No. 2006-42978).
Examples of the test chart presented by the optotype presenting apparatus include a visual acuity test chart used in a refractive power test (see FIG. 1), a red-green test chart for testing overcorrection, a phoria test chart, a fusion test chart, and a stereoscopic vision test chart, which are test charts for testing binocular visual performance by making right and left eyes respectively observe different optotypes. The binocular visual performance test chart includes a red-green binocular visual performance test chart such that different optotypes are presented to right and left eyes of an examinee with a red filter placed in front of one of the eyes and a green filter placed in front of the other eye.
FIGS. 3A to 3D show an example of a Worth four-point chart used for testing fusion and suppression of eyes. In FIG. 3A, a Worth four-point chart (test chart) 100 includes, on a black background 101, a circular red optotype 102 on the upper side, circular green optotypes 104 and 105 respectively on the left side and the right side, and a circular white optotype 103 on the lower side. When the test chart 100 is observed through a red filter placed in front of a right eye, the red optotype 102 and the white optotype 103 are visually perceived in red as shown in FIG. 3B. Meanwhile, the green optotypes 104 and 105 are visually perceived in black and appear merged with the black background 101 because green light therefrom is cut off by the red filter. In addition, when the test chart 100 is observed through a green filter in front of a left eye, the red optotype 102 is visually perceived in black and appears merged with the black background 101 as shown in FIG. 3C because red light therefrom is cut off by the green filter. Meanwhile, the green optotypes 104 and 105 and the white optotype 103 are visually perceived in green. When the test chart 100 is observed by both of the eyes, it is judged that the left eye is suppressed if the optotypes are visually perceived in the state shown in FIG. 3B, and it is judged that the right eye is suppressed if the optotypes are visually perceived in the state shown in FIG. 3C. In addition, if fusion is proper, the red optotype 102 and the green optotypes 104 and 105 are simultaneously visually perceived, and the white optotype 103 is visually perceived alternately in red and green.
For respectively placing the red filter and the green filter in front of the eyes, a subjective refractive power measurement apparatus capable of switching between a red filter and a green filter to be respectively placed in right and left test windows, or red and green spectacles with a red filter and a green filter are used.
However, red filters to be used in the subjective refractive power measurement apparatus and the red and green spectacles have different wavelength transmission characteristics even if they are similarly red. Green filters also have different wavelength transmission characteristics in some cases. These differences result from different apparatus manufacturers, or even when the red filters and/or the green filters are produced by one apparatus manufacturer, there are variations between the red filters and/or between the green filters. Therefore, there is a problem that when a subjective refractive power measurement apparatus or red and green spectacles are used, a green optotype and a red optotype are not visually perceived independently by right and left eyes, and the optotypes do not partly merge with the background and are unintentionally visually perceived by unintended eyes. For example, in the test chart 100 shown in FIG. 3A, when the test chart 100 is observed through the red filter, the green optotypes 104 and 105 do not merge with the background 101 and are unintentionally visually perceived in light green as shown in FIG. 3D. In such a case, the binocular visual performance test cannot be accurately performed.